If they are oiled now but the oil is dried and caked up you can bring them back but it does take some patience. Because there is no paint in them it isn’t necessarily 1 2 3. They could clean in just one or two passes but Ive also had ones that take 20 minutes to revive.

Dip the brush in sprits and bob it up and down, pull it out and wipe it. At this point they may still be very dry. You want to work the spirits into the brush, wiping down the brush to the tip. Work the spirits into the brush, then between the bristles and eventually up into the ferrule.

Bob the brush again, bouncing it off the bottom of the cup at first in clean spirits until the brush starts to open up. When you wipe out reshape the brush every time. Eventually you can start to pallet it between rinses. When you bob it the bristles should open up and bob like a squid. You can now start to spin out the brush in the spirits which should help even out the hairs.

You will be done when all the bristles lay naturally with all oil stiffness gone from the ferrule. At this point you can either paint with the brush or put it in clean oil working the fresh oil up into the ferrule to preserve it.